Aluminum-Based Composite Material and Method for Production Thereof

ABSTRACT

An aluminum base composite material ( 10 ) having an aluminum alloy ( 11 ) as a base material and, incorporated therein, a reinforcing material ( 12 ) composed of alumina particles of alumina fibers, wherein a spinel layer ( 13 ) is formed on the surface of the reinforcing material and an aluminum nitride layer ( 14 ) exhibiting excellent wettability is formed on the surface of said spinel layer.

TECHNICAL FIELD

The present invention relates to an aluminum-based composite material in which an aluminum alloy is used as a base material and in which the base material contains a reinforcing material composed of alumina particles or alumina fibers. The present invention also relates to a method for production of the composite material.

BACKGROUND ART

A composite material containing reinforcing material in a metal material as a base material is used to improve the characteristics of the metal material. An example of such a composite material is an aluminum-based composite material whose base material is aluminum (Al) and whose reinforcing material is alumina (Al₂O₃) particles.

An example of a method for producing an aluminum-based composite material is a method in which an aluminum alloy is allowed to penetrate into a porous compact, as disclosed in, e.g., Japanese Patent No. 2998828.

According to the production method disclosed in this publication, first, a porous compact composed of alumina (Al₂O₃) particles is placed on an aluminum (Al) block, magnesium (Mg) is arranged adjacent to the compact, and these components are heated to 900° C. The heating causes the aluminum block to melt and the magnesium to sublimate.

Next, the components are stored in an atmosphere of nitrogen gas (N₂), and the nitrogen gas reacts with the magnesium to form magnesium nitride (Mg₃N₂). The resulting Mg₃N₂ reduces the Al₂O₃ on the surface of the porous compact and exposes the aluminum on the surface of the porous compact.

The melted aluminum is caused to penetrate into the spaces in the porous compact, and an aluminum-based composite material is obtained.

In the aluminum-based composite material and other composite materials, the reinforcing material contained in the metal material may bring about a chemical reaction with the elements in the metal material. If the reinforcing material chemically reacts with the elements in the metal material, the shape of the reinforcing material changes, and the strength of the composite material is difficult to maintain.

Japanese Laid-open Patent Application No. 2001-316785 discloses, as a countermeasure, a method for producing a composite material comprising a spinel layer on the surface of a reinforcing material.

According to the production method disclosed in Japanese Laid-open Patent Application No. 2001-316785, aluminum borate whiskers (made by Shikoku Chemicals Corporation) are used as the reinforcing material, and the surface of the reinforcing material is first covered with magnesium (Mg).

Next, the reinforcing material is heated in a vacuum at 500 to 1200° C., and this heating is continued for 0.5 to 3 hours.

A chemical reaction takes place between the reinforcing material and the magnesium, resulting in a composite material in which a spinel layer is formed on the surface of the reinforcing material. Forming a spinel layer on the surface of the reinforcing material prevents the reinforcing material contained in the metal material from causing a chemical reaction with the elements in the metal material. The shape of the reinforcing material is thereby prevented from changing due to the chemical reaction, and the strength of the composite material can be maintained.

The production method disclosed in Japanese Laid-open Patent Application No. 2001-316785 describes a composite material in which aluminum borate whiskers are used as the reinforcing material in the above-described manner. Aluminum borate is known as a material that easily forms a spinel.

Therefore, in the production method described above, a spinel layer cannot be formed on the surface of the reinforcing material in cases in which alumina is used instead of aluminum borate as the reinforcing material. Therefore, it has been established that in a composite material in which alumina is used as the reinforcing material, the reinforcing material comes into contact with the elements in the metal material and causes a chemical reaction, and the characteristics of the composite material are difficult to maintain.

Furthermore, in the production method disclosed in Japanese Laid-open Patent Application No. 2001-316785, when the molten aluminum alloy enters (penetrates into) the reinforcing material, sufficient wettability between the aluminum alloy and the spinel layer may not be possible.

Therefore, defects (i.e., voids) may form in the aluminum-based composite material, causing the aluminum-based composite material to acquire local brittleness.

In view of this, there is a need for a technique whereby the characteristics can be improved in a composite material in which alumina is used as a reinforcing material.

DISCLOSURE OF THE INVENTION

The present invention provides an aluminum-based composite material including as a base material an aluminum alloy which contains a reinforcing material composed of alumina particles or alumina fibers, wherein the aluminum-based composite material comprises a spinel layer formed on a surface of the reinforcing material, and an aluminum nitride layer formed on a surface of the spinel layer.

Bonding between the aluminum alloy and the reinforcing material is improved because the spinel layer formed from the reinforcing material and magnesium nitride (Mg₃N₂), the nitrogen produced during formation of the spinel layer, and the molten aluminum alloy react together to form aluminum nitride. In other words, the aluminum alloy can be bonded extremely well to the surface of the reinforcing material by the formation of an aluminum nitride layer on the surface of the spinel layer.

A method for causing a molten aluminum alloy (melt) to penetrate into the reinforcing material is used in order to include the reinforcing material into the aluminum alloy. At this time, the aluminum alloy may come into contact with the reinforcing material, and the aluminum alloy components may react with the reinforcing material.

In view of this, the reinforcing material is prevented from reacting with the elements in the aluminum alloy by forming a uniform spinel layer on the surface of the reinforcing material before the molten aluminum alloy is caused to penetrate into the gaps in the reinforcing material. The characteristics of a composite material in which alumina is used as the reinforcing material can thereby be maintained.

Preferably, the spinel layer contains an auxiliary spinel layer formed in advance on the surface of the reinforcing material. Thus, by forming an auxiliary spinel layer on the surface of the reinforcing material, the thickness of the entire spinel layer can easily be increased, and the aluminum alloy can be better bonded to the surface of the reinforcing material.

Furthermore, the present invention provides a method for producing an aluminum-based composite material in which an aluminum alloy is used as a base material and in which the base material contains a reinforcing material composed of alumina particles or alumina fibers; wherein the method comprises the steps of mixing the reinforcing material with magnesium to form a mixed powder, placing a billet of the aluminum alloy on the mixed powder, sublimating the magnesium in the mixed powder by heating the billet and the mixed powder to the sublimation temperature of magnesium in a nitrogen atmosphere, forming a spinel layer on the surface of the reinforcing material by reacting the alumina of the reinforcing material with the magnesium nitride produced from a reaction between the sublimated magnesium and the nitrogen, and forming aluminum nitride on the surface of the spinel layer while raising the temperature to the melting point of the billet and causing the molten aluminum alloy to penetrate into the mixed powder after the spinel layer is formed.

The spinel layer is formed on the surface of the reinforcing material in the nitrogen atmosphere. Thus, using a nitrogen atmosphere makes it possible to satisfactorily form a spinel layer on the surface of the reinforcing material even if alumina is used as the reinforcing material.

Furthermore, since the reinforcing material is mixed with magnesium to form a mixed powder, sublimation of the magnesium sublimates allows the sublimated magnesium to envelop the reinforcing material. This magnesium reacts with the nitrogen to produce magnesium nitride. Accordingly, a large amount of magnesium nitride is produced around the reinforcing material. The magnesium nitride thereby reacts satisfactorily with the alumina of the reinforcing material, and a uniform spinel layer is formed on the surface of the reinforcing material.

Thus, the reinforcing material is prevented from reacting with the elements in the aluminum alloy by the formation of a uniform spinel layer on the surface of the reinforcing material before the molten aluminum alloy is caused to penetrate into the mixed powder. Therefore, the characteristics of a composite material in which alumina is used as a reinforcing material can be maintained.

Furthermore, an aluminum nitride layers is formed on the surface of the spinel layer. Aluminum nitride has excellent wettability. Defects (i.e., voids) can thereby be prevented in the aluminum-based composite material, and the aluminum alloy can be bonded extremely well to the surface of the reinforcing material.

In the step for sublimating the magnesium, the temperature is preferably raised to the sublimation temperature of magnesium and then kept at this sublimation temperature. The magnesium therefore sublimates gradually. The surface of the reinforcing material is thereby allowed retain the magnesium for a comparatively long period of time, a uniform spinel layer can be formed on the surface of the reinforcing material, and the strength of the aluminum-based composite material can be more reliably ensured.

A step for forming an auxiliary spinel layer in advance on the surface of the reinforcing material is preferably included before the step for forming the mixed powder.

Thus, forming an auxiliary spinel layer in advance on the surface of the reinforcing material makes it possible to easily increase the thickness of the entire spinel layer. The aluminum alloy can thereby be even better bonded to the surface of the reinforcing material. The thickness of the entire spinel layer can be controlled, and a composite material that conforms to the desired material characteristics can easily be obtained.

In addition, more reliably forming a spinel layer on the surface of the reinforcing material before the aluminum alloy melt penetrates into the metal powder makes it possible to more reliably prevent the reinforcing material from reacting with the elements in the aluminum alloy, and to more appropriately maintain the characteristics of the aluminum-based composite material.

A powder having a grain size of 50 to 500 μm is preferred for the magnesium.

The magnesium can be added in bulk form or as a constituent element of an aluminum alloy. However, adding magnesium in bulk may prevent the magnesium from being sublimated and leave a magnesium residue, compromising the material characteristics. Adding magnesium as a component of an aluminum alloy creates differences in the quantitative relationship or distance between the reinforcing material and the magnesium in the aluminum alloy, and uniform distribution becomes difficult.

In view of this, in the present invention, a powder having a grain size of about 50 to 500 μm is used for the magnesium as previously described. Magnesium powder having a grain size of 50 to 500 μm has a small grain size and will therefore easily sublimate at temperatures equal to or less than the sublimation temperature of magnesium; for example, at 550° C. Specifically, the sublimation temperature of magnesium can be kept at 550° C.

If the grain size of the magnesium is less than 50 μm, the magnesium powder reacts too easily, the magnesium oxidizes merely from exposure to air to form magnesium oxide, and the magnesium is difficult to supply in sufficient amounts for the reactions of the present invention.

In cases in which the grain size of the magnesium exceeds 500 μm, magnesium that cannot sublimate may remain in the composite material, similar to the case of magnesium in bulk form.

Magnesium powder having a grain size of 50 to 500 μm is therefore used in the present invention. The magnesium powder can thereby be satisfactorily sublimated at the desired sublimation temperature (550° C.), and a uniform spinel layer is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an aluminum-based composite material according to a first embodiment of the present invention;

FIG. 2 is a flowchart showing a method for producing an aluminum-based composite material according to the first embodiment;

FIGS. 3A and 3B are schematic views showing an example of mixing the reinforcing material with magnesium powder in the method for producing an aluminum-based composite material according to the first embodiment;

FIGS. 4A through 4C are schematic views showing an example of forming a spinel layer on the surface of the reinforcing material in the method for producing an aluminum-based composite material according to the first embodiment;

FIGS. 5A and 5B are schematic views showing an example in which the temperature is raised to the melting point of the aluminum alloy billet in the method for producing an aluminum-based composite material according to the first embodiment;

FIG. 6 is a diagram showing an example in which a molten aluminum alloy penetrates into the spaces in a porous compact in the method for producing an aluminum-based composite material according to the first embodiment;

FIG. 7 is a cross-sectional view showing an aluminum-based composite material according to a second embodiment of the present invention;

FIG. 8 is an enlarged view from a photograph of the aluminum-based composite material according to the second embodiment;

FIG. 9 is a flowchart showing the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 10A and 10B are schematic views showing an example of forming an auxiliary spinel layer in advance on the surface of the reinforcing material in the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 11A and 11B are schematic views showing an example of mixing magnesium powder having the reinforcing material whose surface is coated with an auxiliary spinel layer in the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 12A and 12B are schematic views showing an example of sublimating the magnesium powder in the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 13A and 13C are schematic views showing an example of forming a spinel layer on the surfaces of the an auxiliary spinel layer in the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 14A and 14B are schematic views showing an example of causing the molten aluminum alloy to penetrate into the spaces in a porous compact in the method for producing an aluminum-based composite material according to the second embodiment;

FIGS. 15A through 15E are schematic views showing a test sample according to a comparative example of an aluminum-based composite material, and test samples according to Embodiments 1 through 4; and

FIG. 16 is a graph showing the flexural strength of the test samples shown in FIGS. 15A through 15E.

BEST MODE FOR CARRYING OUT THE INVENTION

Certain preferred embodiments of the present invention are described below with reference to the accompanying drawings.

FIG. 1 shows an aspect of the aluminum-based composite material according to the first embodiment of the present invention.

An aluminum-based composite material 10 has an aluminum alloy 11 as a base material, and the base material contains a reinforcing material 12 composed of alumina particles or alumina fibers. A spinel layer 13 is formed on the surface 12 a of the reinforcing material 12, and an aluminum nitride layer 14 is formed on the surface 13 a of the spinel layer 13.

Spherical alumina (Al₂O₃) particles are shown as an example of the reinforcing material 12, but the shape of the reinforcing material 12 is not limited to spheres and can be determined arbitrarily. For example, the same effects can be obtained when alumina fibers are used instead of alumina particles.

The spinel layer 13 is composed of an oxide (MgAl₂O₄) produced by a reaction between alumina (Al₂O₃) and magnesium nitride (Mg₃N₂). The spinel layer 13 is preferably formed with a uniform thickness t1 over the entire surface 12 a of the reinforcing material 12.

The aluminum nitride layer 14 is formed when the molten aluminum alloy 11 s and penetrates into the spaces of the reinforcing material 12, and nitrogen (N₂) reacts with the aluminum (Al) components in the aluminum alloy 11. The aluminum nitride has excellent wettability.

Next, the method for producing the aluminum-based composite material 10 according to the first embodiment shown in FIG. 1 will be described with reference to FIGS. 2 through 6.

FIG. 2, step (hereinafter abbreviated as ST) 10: The reinforcing material is mixed with magnesium (Mg) powder to form a porous compact (mixed powder).

ST11: A billet of the aluminum alloy is placed on the porous compact.

ST12: The billet and porous compact are heated to the sublimation temperature of the magnesium powder in a nitrogen atmosphere, and the magnesium powder in the porous compact is sublimated.

ST13: The sublimated magnesium powder reacts with the nitrogen (N₂) to produce magnesium nitride (Mg₃N₂). This magnesium nitride reacts with the reinforcing material alumina (Al₂O₃) to form a spinel layer on the surface of the reinforcing material.

ST14: After the spinel layer is formed, the temperature is raised to the melting point of the billet, and the molten aluminum alloy (aluminum alloy melt) is caused to penetrate into the porous compact, forming aluminum nitride on the surface of the spinel layer.

The aluminum-based composite material 10 shown in FIG. 1 is obtained through the steps described above.

ST10 through ST14 are described in detail hereinbelow with reference to FIGS. 3A through 6.

FIGS. 3A and 3B show an example of mixing magnesium powder having the reinforcing material described in ST10 of FIG. 2.

A lid 21 on a mixing container 20 is removed to open an opening (not shown) in the mixing container 20, as shown in FIG. 3A. The reinforcing material (alumina particles) 12 and magnesium (Mg) powder 15 as magnesium are introduced into the mixing container 20 through this opening. The magnesium powder 15 has a grain size of 50 to 500 μm.

The opening in the mixing container 20 is closed with the lid, and the mixing container 20 is attached to a stirring apparatus (not shown). The mixing container 20 is held in the stirring apparatus and rotated as shown by the arrow A, whereby the reinforcing material 12 and the magnesium powder 15 are mixed.

After mixing, the reinforcing material 12 and the magnesium powder 15 are removed from the mixing container 20.

In FIG. 3B, a porous compact (mixed powder) 16 is obtained from the extracted reinforcing material 12 and magnesium powder 15. In this porous compact 16, the magnesium powder 15 adheres substantially uniformly to the surface 12 a of the reinforcing material 12.

FIGS. 4A through 4C show an example of forming a spinel layer on the surface of the reinforcing material in the method for producing an aluminum-based composite material according to the first embodiment, wherein FIG. 4A(a) shows a specific example of ST11 and ST12 in FIG. 2, and FIGS. 4B and 4C show a specific example of ST13 in FIG. 2.

In FIG. 4A, the porous compact 16 is placed on the bottom surface 27 a of a crucible 27 in an atmosphere furnace 26 constituting a production apparatus 25 for the aluminum-based composite material, and a billet 17 of the aluminum alloy is placed on the porous compact 16.

To remove the air from the atmosphere furnace 26, vacuum suction is applied with a vacuum pump 29, and the vacuum pump 29 is stopped when a predetermined vacuum is reached. Argon gas (Ar) is then supplied into the atmosphere furnace 26. An atmosphere of argon gas is formed inside the atmosphere furnace 26, and the aluminum alloy billet 17 and magnesium powder 15 are prevented from oxidizing.

Next, nitrogen gas (N₂) 32 is supplied to the atmosphere furnace 26 as shown by the arrow B. At the same time, the pressure inside the atmosphere furnace 26 is increased (e.g., atmospheric pressure+approx. 0.5 kg/cm²), and the atmosphere in the atmosphere furnace 26 is replaced with the nitrogen gas 32.

Next, the atmosphere furnace 26 is heated with a heating coil 34, whereby the porous compact 16 and the aluminum alloy billet 17 are heated to the sublimation temperature of the magnesium powder 15 (e.g., about 550° C.). The temperature in the atmosphere furnace 26 is sensed with a temperature sensor 35, and the temperature in the atmosphere furnace 26 is kept at the sublimation temperature (e.g., about 550° C.) by a controller 36 on the basis of a sensor signal from the temperature sensor 35.

The sublimation time of the magnesium powder 15 is about three hours.

In FIG. 4B, the sublimated magnesium 38 reacts with the nitrogen gas 32 to produce magnesium nitride (Mg₃N₂) 41

The resulting magnesium nitride (Mg₃N₂) 41 reacts as follows with the alumina (Al₂O₃) in the reinforcing material 12 that constitutes the porous compact 16. 3Al₂O₃+Mg₃N₂+3[O]→3MgAl₂O₄+N₂

In FIG. 4C, the magnesium nitride (Mg₃N₂) 41 reacts with the alumina in the reinforcing material 12, whereby a spinel (MgAl₂O₄) layer 13 is formed over the entire surface 12 a of the reinforcing material 12.

As described above, the sublimation temperature of the magnesium powder 15 is maintained (e.g., about 550° C.), whereby the magnesium powder 15 is gradually sublimated. The magnesium powder 15 is thereby allowed to remain on the surface 12 a of the reinforcing material 12 for a comparatively long period of time, and a uniform spinel layer 13 is formed over the entire surface 12 a of the reinforcing material 12.

The reasons for forming the spinel (MgAl₂O₄) layer 13 on the surface 12 a of the reinforcing material 12 will now be described.

The reinforcing material 12 is mixed with the magnesium powder 15 to form the porous compact 16, as shown in FIG. 3B. Accordingly, when the magnesium powder 15 sublimates, the sublimated magnesium 38 envelops the reinforcing material 12, as shown in FIG. 4B. This magnesium 38 reacts with the nitrogen 32 to produce magnesium nitride 41.

Large amounts of magnesium nitride 41 are thereby produced around the reinforcing material 12, as shown in FIG. 4B. The magnesium nitride 41 is thereby caused to react with the alumina in the reinforcing material 12 to form the spinel layer 13 over the entire surface 12 a of the reinforcing material 12.

FIGS. 5A and 5B show an example in which the temperature is raised to the melting point of the aluminum alloy billet in the method for producing an aluminum-based composite material according to the first embodiment. FIG. 5A shows a specific example of ST13 in FIG. 2, and FIG. 5B shows a specific example of ST14 in FIG. 2.

In the porous compact 16, the magnesium powder 15 is disposed substantially uniformly over the surface 12 a of the reinforcing material 12, as shown in FIG. 3B. Accordingly, the spinel layer 13 is formed uniformly over the entire surface 12 a of the reinforcing material 12, as shown in FIG. 5A.

In FIG. 5B, after the uniform spinel layer 13 has been formed over the surface 12 a of the reinforcing material 12, the temperature is raised to the melting point (850° C.) of the aluminum alloy billet 17. The aluminum alloy billet 17 melts, and the molten aluminum alloy 11 (see FIG. 1) penetrates into the porous compact 16, as shown by the arrow C.

Since the melt penetrates into the porous compact 16 as shown by the arrow C after the spinel layer 13 has been uniformly formed over the surface 12 a of the reinforcing material 12, the reinforcing material 12 is prevented from reacting with the elements in the aluminum alloy 11.

FIG. 6 shows an example in which the material of the aluminum alloy billet penetrates into the spaces in the porous compact in the method for producing an aluminum-based composite material according to the first embodiment, and depicts ST14 of FIG. 2 in detail.

The molten aluminum alloy 11 (see FIG. 1) penetrates into the spaces 16 a of the porous compact 16 as shown by the arrow D.

The aluminum (Al) components in the molten aluminum alloy 11 react with the nitrogen (N₂). An aluminum nitride (AlN) layer 14 is formed by this reaction on the surface 13 a of the spinel layer 13.

Aluminum nitride has excellent wettability. Accordingly, the molten aluminum alloy 11 satisfactorily fills in the spaces 16 a in the porous compact 16. The aluminum-based composite material 10 shown in FIG. 1 is thereby obtained.

Returning to FIG. 1, the aluminum-based composite material 10 has a reinforcing material 12, a spinel layer 13 formed from magnesium nitride 41 (see FIG. 4B), and an aluminum nitride layer 14 formed by a reaction between the nitrogen produced during the formation of the spinel layer 13 and the molten aluminum alloy 11.

Accordingly, bonding is improved between the aluminum alloy 11 and the reinforcing material 12, and a base material, i.e., the aluminum alloy 11, can be bonded extremely well to the surface 12 a of the reinforcing material 12.

Additionally, the reinforcing material 12 is prevented from reacting with the elements in the aluminum alloy 11 by forming a uniform spinel layer 13 on the surface 12 a of the reinforcing material 12 before the molten aluminum alloy 11 penetrates into the spaces 16 a (see FIG. 6) of the porous compact 16. Therefore, the characteristics of the aluminum-based composite material 10 in which alumina is used as the reinforcing material 12 can be maintained.

Furthermore, an aluminum nitride layer 14 is formed on the surface 13 a of the spinel layer 13. Aluminum nitride has excellent wettability. Accordingly, sufficient wettability can be ensured between the aluminum alloy 11 and the aluminum nitride layer 14 when the molten aluminum alloy 11 penetrates into the spaces of the reinforcing material 12. Defects (i.e., voids) are thereby prevented from forming in the aluminum-based composite material 10, the aluminum alloy 11 can be bonded extremely well to the reinforcing material 12, and the characteristics of the aluminum-based composite material 10 can be improved.

Next, the aluminum-based composite material of the second embodiment, as well as the production method thereof, will be described with reference to FIGS. 7 through 14A. In the aluminum-based composite material of the second embodiment, materials and components identical to or resembling those of the aluminum-based composite material 10 of the first embodiment are denoted by the same numerical symbols and are not described.

First, the aluminum-based composite material of the second embodiment will be described with reference to FIGS. 7 and 8.

An aluminum-based composite material 50 has an aluminum alloy 11 as a base material, and the base material contains a reinforcing material 12 composed of alumina particles or alumina fibers, wherein an auxiliary spinel layer 51 is formed on the surface 12 a of the reinforcing material 12, a spinel layer 13 is formed on the surface 51 a of the auxiliary spinel layer 51, and an aluminum nitride layer 14 is formed on the surface 13 a of the spinel layer 13.

Similar to the spinel layer 13, the auxiliary spinel layer 51 is made of an oxide (MgAl₂O₄) produced by a reaction between alumina (Al₂O₃) and magnesium nitride (Mg₃N₂). The auxiliary spinel layer 51 is preferably formed with a uniform thickness t2 over the entire surface 12 a of the reinforcing material 12.

Next, the method for producing the aluminum-based composite material 50 of the second embodiment will be described with reference to FIGS. 9 through 14B. FIG. 9 is a flowchart showing the method for producing an aluminum-based composite material according to the second embodiment.

ST20: An auxiliary spinel layer is formed in advance on the surface of the reinforcing material.

ST21: The reinforcing material having the auxiliary spinel layer formed in advance is mixed with magnesium (Mg) powder to form a porous compact (mixed powder).

ST22: A billet of the aluminum alloy is placed on top of the porous compact.

ST23: The billet and the porous compact are heated in a nitrogen atmosphere to the sublimation temperature of the magnesium powder, and the magnesium powder in the porous compact is sublimated.

ST24: The sublimated magnesium powder reacts with the nitrogen (N₂) to produce magnesium nitride (Mg₃N₂). This magnesium nitride reacts with the alumina (Al₂O₃) in the reinforcing material to form a spinel layer on the surface of the reinforcing material.

ST25: After the spinel layer is formed, the temperature is raised to the melting point of the billet, and the molten aluminum alloy penetrates into the porous compact while aluminum nitride is formed on the surface of the spinel layer.

The aluminum-based composite material 50 shown in FIG. 7 is thereby obtained.

ST20 through ST24 will now be described in detail with reference to FIGS. 10A through 14B.

FIGS. 10A and 10B show an example of forming an auxiliary spinel layer in advance on the surface of the reinforcing material, and depict ST20 of FIG. 9 in detail.

In FIG. 10A, the reinforcing material 12 is mixed with magnesium hydroxide (Mg(OH)₂) 52. The magnesium hydroxide 52 is thereby arranged substantially uniformly over the surface 12 a of the reinforcing material 12.

In FIG. 10B, the materials are heated for five hours at the reaction temperature (1000 to 1500° C.) of the spinel layer in a normal atmosphere, an inert atmosphere, or a vacuum.

The magnesium hydroxide (Mg(OH)₂) 52 reacts with the alumina (Al₂O₃) in the reinforcing material 12. A preparatory spinel (MgAl₂O₄) layer 51 is formed by the reaction in a uniform thickness t2 over the entire surface 12 a of the reinforcing material 12.

FIGS. 11A and 11B show an example of mixing the reinforcing material with magnesium powder, and depict part of ST21 of FIG. 9.

In FIG. 11A, a uniform auxiliary spinel layer 51 is formed on the entire surface 12 a of the reinforcing material 12.

In FIG. 11B, a lid 21 on a mixing container 20 is removed to open an opening (not shown) in the mixing container 20. The magnesium (Mg) powder 15 and the reinforcing material (alumina particles) 12 provided with the auxiliary spinel layer 51 are introduced into the mixing container 20 through the opening.

The opening in the mixing container 20 is closed with the lid 21, and the mixing container 20 is attached to a stirring apparatus (not shown). The mixing container 20 is rotated as shown by the arrow E, whereby the magnesium (Mg) powder 15 and the reinforcing material (alumina particles) 12 provided with the auxiliary spinel layer 51 are mixed.

The magnesium powder 15 and the reinforcing material 12 provided with the auxiliary spinel layer 51 are then taken out from the mixing container 20.

FIGS. 12A and 12B show an example of a sublimating magnesium powder, wherein FIG. 12A shows part of ST21 of FIG. 9 in detail, and FIG. 12B shows ST22 and ST23 in detail.

In FIG. 12A, the extracted reinforcing material 12 and magnesium powder 15 are formed into a pulverulent substance to obtain a porous compact (mixed powder) 54. In the porous compact 54, the magnesium powder 15 is disposed substantially uniformly over the surface 51 a of the auxiliary spinel layer 51.

In FIG. 12B, the porous compact 54 is disposed on the bottom surface 27 a of a crucible 27 in an atmosphere furnace 26 constituting the production apparatus 25 for the aluminum-based composite material, and an aluminum alloy billet 17 is placed on the porous compact 54.

To remove the air from the atmosphere furnace 26, vacuum suction is applied with a vacuum pump 29, and the vacuum pump 29 is stopped when a predetermined vacuum is reached. Argon gas (Ar) is then supplied into the atmosphere furnace 26. The result is an atmosphere of argon gas inside the atmosphere furnace 26, and the aluminum alloy billet 17 and magnesium powder 15 are prevented from oxidizing.

Next, nitrogen gas (N₂) 32 is supplied to the atmosphere furnace 26, as shown by the arrow F. At the same time, the pressure inside the atmosphere furnace 26 is increased (e.g., atmospheric pressure+approx. 0.5 kg/cm²), and the atmosphere in the atmosphere furnace 26 is replaced with the nitrogen gas 32.

Next, the atmosphere furnace 26 is heated with a heating coil 34, whereby the porous compact 54 and the aluminum alloy billet 17 are heated to the sublimation temperature of the magnesium powder 15 (e.g., about 550° C.). The temperature in the atmosphere furnace 26 is sensed with a temperature sensor 35, and the temperature in the atmosphere furnace 26 is kept at the sublimation temperature (e.g., about 550° C.) by a controller 36 on the basis of a sensor signal from the temperature sensor 35.

FIGS. 13A through 13C show an example of forming a spinel layer on the surfaces of the auxiliary spinel layer, and depict ST24 of FIG. 9 in detail.

In FIG. 13A, the sublimated magnesium 38 reacts with the nitrogen gas 32 to produce magnesium nitride (Mg₃N₂) 41. The resulting magnesium nitride (Mg₃N₂) 41 reacts with the alumina (Al₂O₃) components in the preparatory spinel (MgAl₂O₄) layer 51 as follows. 3Al₂O₃+Mg₃N₂+3[O]→3MgAl₂O₄+N₂

In FIG. 13B, the magnesium nitride 41 reacts with the alumina components in the auxiliary spinel layer 51, whereby a spinel (MgAl₂O₄) layer 13 is formed over the entire surface 51 a of the auxiliary spinel layer 51.

As described above, the magnesium powder 15 is gradually sublimated by maintaining the sublimation temperature of the magnesium powder 15 (e.g., about 550° C.). The magnesium powder 15 is thereby allowed to remain on the surface 51 a of the auxiliary spinel layer 51 for a comparatively long period of time, and a uniform spinel layer 13 to be formed over the entire surfaces 51 a, as shown in FIG. 13C.

The reasons for forming a spinel (MgAl₂O₄) layer on the surface 51 a of the auxiliary spinel layer 51 will now be described.

The reinforcing material 12 provided with the auxiliary spinel layer 51 is mixed with the magnesium powder 15 to form the porous compact 54, as shown in FIG. 12A. Accordingly, when the magnesium powder 15 sublimates, the sublimated magnesium 38 envelops the auxiliary spinel layer 51, as shown in FIG. 13A. The magnesium 38 reacts with the nitrogen 32 to produce magnesium nitride 41. Large amounts of magnesium nitride 41 are produced around the auxiliary spinel layer 51, as shown in FIG. 13A. The magnesium nitride 41 reacts with the alumina component in the auxiliary spinel layer 51 to form a spinel layer 13 over the entire surface 51 a of the auxiliary spinel layer 51.

A uniform spinel layer 13 is formed over the entire surface 51 a of the auxiliary spinel layer 51, as shown in FIG. 13C.

FIGS. 14A and 14B show an example in which the material of the aluminum alloy billet penetrates into the spaces in the porous compact, and depict ST25 in FIG. 9 in detail.

In FIG. 14A, after the uniform spinel layer 13 has been formed on the surface 51 a of the auxiliary spinel layer 51, the materials are heated to the melting point (850° C.) of the aluminum alloy billet 17. The aluminum alloy billet 17 melts, and the molten aluminum alloy 11 (see FIG. 7) penetrates into the porous compact 54, as shown by the arrow G. Therefore, the reinforcing material 12 is prevented from reacting with the elements in the aluminum alloy 11.

In FIG. 14B, the molten aluminum alloy 11 (see FIG. 7) penetrates into the spaces 54 a of the porous compact 54, as shown by the arrow H. The aluminum (Al) components of the molten aluminum alloy 11 react with the nitrogen (N₂). An aluminum nitride (AlN) layer 14 is formed by the reaction on the surface 13 a of the spinel layer 13.

Aluminum nitride has excellent wettability. Accordingly, the molten aluminum alloy 11 satisfactorily fills in the spaces 54 a of the porous compact 54, and the aluminum-based composite material 50 shown in FIG. 7 is obtained.

Returning to FIGS. 7 and 8, in the aluminum-based composite material 50, the magnesium nitride 41 (see FIG. 13A) reacts with the alumina components in the auxiliary spinel layer 51 to form a spinel layer 13 on the surface 51 a of the auxiliary spinel layer 51. Accordingly, the auxiliary spinel layer 51 and the spinel layer 13 can be formed on the surface 12 a of the reinforcing material 12, and the thickness of the spinel layers 51, 13 can be increased to (t2+t1), as shown in FIG. 13B. The aluminum alloy 11 as a base material can thereby be bonded even more satisfactorily to the surface 12 a of the reinforcing material 12.

Thus, the reinforcing material 12 can be prevented even more satisfactorily and reliably from reacting with the elements in the aluminum alloy 11. This is achieved by further increasing the thickness (t2+t1) of the spinel layers 51, 13 before the molten aluminum alloy 11 penetrates into the spaces 54 a of the porous compact 54.

Therefore, the characteristics of the aluminum-based composite material 50 can be even more appropriately maintained.

Furthermore, forming an auxiliary spinel layer 51 in advance makes it possible to control the thickness of the spinel layers 51, 13, and to easily obtain a composite material that conforms to the desired material characteristics.

Similar to the aluminum-based composite material 10 of the first embodiment shown in FIG. 1, forming an aluminum nitride layer 14 on the surface 13 a of the spinel layer 13 makes it possible to prevent the occurrence of defects (i.e., voids) in the aluminum-based composite material 50. The aluminum alloy 11 can thereby be bonded extremely well to the reinforcing material 12, and the characteristics of the aluminum-based composite material 50 can be improved.

Next, the characteristics (particularly flexural strength) of the aluminum-based composite materials 10, 50 of Embodiments 1 and 2 will be described.

FIGS. 15A through 15E show a test sample of the aluminum-based composite material, wherein FIG. 15A shows a comparative example, and FIGS. 15B through 15E show Embodiments 1 through 4.

The comparative example shown in FIG. 15A is an aluminum-based composite material in which a reinforcing material 12 is contained an aluminum alloy 11 in a state in which a spinel layer is not formed on the reinforcing material 12 composed of alumina particles. Alternatively, a material in which magnesium is contained as an elemental component in the aluminum alloy 11 may be caused to penetrate into the gaps in the reinforcing material 12 composed of alumina particles, producing in an aluminum-based composite material in which [the reinforcing material] is contained in the aluminum alloy 11 in a state in which a uniform spinel layer is not formed on the reinforcing material 12 composed of alumina particles.

Specifically, in the comparative example, the spinel layer is either not formed or is formed in a nonuniform manner.

In a material in which a spinel layer is not formed in a uniform manner, some regions on the spinel layer have insufficient thickness. The radius R of the reinforcing material 12 is 3 μm.

Embodiment 1 shown in FIG. 15B is an aluminum-based composite material wherein a uniform spinel layer 13 is formed on the reinforcing material 12 composed of alumina particles, an aluminum nitride layer 14 is formed on the spinel layer 13, and the reinforcing material 12 composed of alumina particles is contained in the aluminum alloy 11.

Embodiment 2 shown in FIG. 15C is an aluminum-based composite material wherein a uniform auxiliary spinel layer 51 and spinel layer 13 are formed on the reinforcing material 12 composed of alumina particles, an aluminum nitride layer 14 is formed on the spinel layer 13, and the reinforcing material 12 composed of alumina particles is contained in the aluminum alloy 11. The radius R of the reinforcing material 12 is 3 μm, and the thickness t2 of the auxiliary spinel layer 51 is 25 nm. The volume ratio of the auxiliary spinel layer 51 to the reinforcing material 12 of radius R (3 μm) is 5 vol %.

Embodiment 3 shown in FIG. 15D is an aluminum-based composite material wherein a uniform auxiliary spinel layer 51 and spinel layer 13 are formed on the reinforcing material 12 composed of alumina particles, an aluminum nitride layer 14 is formed on the spinel layer 13, and the reinforcing material 12 composed of alumina particles is contained in the aluminum alloy 11. The radius R of the reinforcing material 12 is 3 μm, and the thickness t2 of the auxiliary spinel layer 51 is 52 nm. The volume ratio of the auxiliary spinel layer 51 to the reinforcing material 12 of radius R (3 μm) is 10 vol %.

Embodiment 4 shown in FIG. 15E is an aluminum-based composite material wherein a uniform auxiliary spinel layer 51 and spinel layer 13 are formed on the reinforcing material 12 composed of alumina particles, an aluminum nitride layer 14 is formed on the spinel layer 13, and the reinforcing material 12 composed of alumina particles is contained in the aluminum alloy 11. The radius R of the reinforcing material 12 is 3 μm, and the thickness t2 of the auxiliary spinel layer 51 is 168 nm. The volume ratio of the auxiliary spinel layer 51 to the reinforcing material 12 of radius R (3 μm) is 30 vol %.

The test samples in the comparative example and in Embodiments 1 through 4 were bent in bending tests to measure the load and deflection of the test samples and to determine flexural strength σ.

The flexural strength σ was determined using the following mathematical formula. σ=M/Z

In the formula, M is bending moment, and Z is the section modulus

Additionally, the Young's modulus E for the test samples in the comparative example and in Embodiments 1 through 4 was determined. The Young's modulus E was determined using the following mathematical formula. E=(P×L)/(λ×A)

In the formula, P is the axial load applied to a test sample, L is the original length of the test sample, λ is stretching or compression of the test sample, and A is the transverse cross-sectional area of the test sample

The results of testing the test samples are shown in the graph in FIG. 16. In this graph, the flexural strength σ (MPa) is plotted on the vertical axis, and the Young's modulus E (GPa) is plotted on the horizontal axis.

In the graph in FIG. 16, the comparative example is shown by ×, Embodiment 1 is shown by O, Embodiment 2 is shown by *, Embodiment 3 is shown by Δ, and Embodiment 4 is shown by □.

In the comparative example, the flexural strength σ had an average of 220 (MPa), and the Young's modulus E had an average of 92 (GPa).

In Embodiment 1, the flexural strength σ had an average of 330 (MPa), and the Young's modulus E had an average of 98 (GPa).

In Embodiment 2, the flexural strength σ had an average of 350 (MPa), and the Young's modulus E had an average of 118 (GPa).

In Embodiment 3, the flexural strength σ had an average of 370 (MPa), and the Young's modulus E had an average of 114 (GPa).

In Embodiment 4, the flexural strength σ had an average of 400 (MPa), and the Young's modulus E had an average of 112 (GPa).

In the comparative example, the flexural strength σ had an average of 220 (MPa), and the Young's modulus E had an average of 92 (GPa), but in Embodiments 1 through 4, the flexural strength σ was 220 (MPa) or greater, and the Young's modulus E was 92 (GPa) or greater.

It is thereby made apparent that forming a spinel layer 13 or 51 and an aluminum nitride layer 14 on the surface 12 a of the reinforcing material 12 makes it possible to improve the characteristics of the aluminum-based composite material.

In the comparative example, there were nonuniformities in the flexural strength σ and the Young's modulus E as shown in the graph, and the reasons for this are as follows.

In the comparative example, either spinel layer were not formed on the reinforcing material 12 composed of alumina particles, or the spinel layer had regions of insufficient thickness.

Specimens in which a spinel layer was not formed could be easily damaged, and damage was likely to propagate from regions of insufficient thickness in specimens in which the thickness of the spinel layer was insufficient.

Therefore, the strength of the composite material is believed to be nonuniform.

The flexural strength σ and Young's modulus E in Embodiments 2 through 4 were greater than in Embodiment 1. It is clear that the characteristics of the aluminum-based composite material can be improved by providing the reinforcing material 12 with an auxiliary spinel layer 51, thereby increasing the thickness of the entire spinel layer to (t1+t2).

Furthermore, it is clear from Embodiments 2 through 4 that the characteristics of the aluminum-based composite material can be improved by increasing the thickness t2 of the auxiliary spinel layer 51 and increasing the thickness (t1+t2) of the entire spinel layer.

The volume ratio of the auxiliary spinel layer 51 in relation to the reinforcing material 12 is preferably 5 vol % or greater and 30 vol % or less.

This is because it is difficult to obtain an adequate effect in a case in which the volume ratio of the auxiliary spinel layer 51 is less than 5 vol %, even if the auxiliary spinel layer 51 is formed.

This is also because increasing the volume ratio of the auxiliary spinel layer 51 above 30 vol % causes too much time to be spent forming the auxiliary spinel layer 51 and impedes productivity.

In the first and second embodiments, examples were described in which porous compacts 16, 54 were used as the mixed powder, but the present invention is not limited to this option alone, and the same effects can be obtained if a powder obtained by mixing the reinforcing material 12 and the magnesium powder 15 is merely placed without modification inside the crucible 27.

In the second embodiment, a method was used wherein magnesium hydroxide 52 was disposed on the reinforcing material 12 and subjected to heat treatment for five hours at the reaction temperature (1000 to 1500° C.) of the spinel layer in order to form an auxiliary spinel layer 51 on the reinforcing material 12. However, the present invention is not limited to this option alone, and magnesium can be used instead of the magnesium hydroxide 52, as in the first embodiment, for example.

Furthermore, in the second embodiment, an example was described in which an auxiliary spinel layer 51 and a spinel layer 13 were formed separately as two layers on the reinforcing material 12 to increase the thickness of the spinel layer, but the present invention is not limited to this option alone, and the thickness of the spinel layer can be increased by increasing the duration of ST13 in FIG. 2 of the first embodiment, or the amount of added magnesium powder 15.

INDUSTRIAL APPLICABILITY

As described above, the present invention can be applied to an aluminum-based composite material in which an aluminum alloy is used as a base material, and in which the base material contains a reinforcing material composed of alumina particles or alumina fibers, and to a method for producing the aluminum-based composite material. 

1. An aluminum-based composite material having as a base material an aluminum alloy which contains a reinforcing material composed of alumina particles or alumina fibers, the aluminum-based composite material comprising: a spinel layer formed on a surface of the reinforcing material; and an aluminum nitride layer formed on a surface of the spinel layer.
 2. The aluminum-based composite material of claim 1, wherein the spinel layer has an auxiliary spinel layer formed in advance on the surface of the reinforcing material.
 3. A method for producing an aluminum-based composite material having as a base material an aluminum alloy which contains a reinforcing material composed of alumina particles or alumina fibers, the method comprising the steps of: mixing the reinforcing material with magnesium to form a mixed powder; placing a billet of the aluminum alloy on the mixed powder; sublimating the magnesium in the mixed powder by heating the billet and the mixed powder to the sublimation temperature of magnesium in a nitrogen atmosphere; forming a spinel layer on the surface of the reinforcing material by reacting the alumina of the reinforcing material with the magnesium nitride produced from a reaction between the sublimated magnesium and the nitrogen; and forming aluminum nitride on the surface of the spinel layer while raising the temperature to the melting point of the billet and causing the molten aluminum alloy to penetrate into the mixed powder after the spinel layer is formed.
 4. The method for producing an aluminum-based composite material, according to claim 3, wherein the sublimation temperature of magnesium is maintained following heating to the sublimation temperature in the step of sublimating the magnesium.
 5. The method for producing an aluminum-based composite material, according to claim 3, further comprising a step of forming auxiliary spinel layer in advance on the surface of the reinforcing material before the step of forming the mixed powder.
 6. The method for producing an aluminum-based composite material, according to claim 3, wherein the magnesium is a powder having a grain size of 50 to 500 μm. 